Once you have reached the speed, the only energy needed is to overcome rolling friction (wheels: bearing friction + rolling resistance on the road) and air drag. Of those, only the rolling friction (F_r) depends on the weight of the "object", the mass enters proportional:
F_r = m \gamma v,
m: mass, \gamma: friction coefficient, v: velocity (first order, but good enough).
an 80 kg rider would need to overcome about 9% higher rolling resistance at the same speed on the heavier bike.
However: the cyclists' main "enemy" is air drag. There, the velocity enters squared, so double speed = four times air drag. It depends also on the surface and shape of the object that is "dragged" through the air, so a more aerodynamical seat position will be more important than a light bike (the velocity needed here is the relative air speed, not the speed over ground, as everybody can tell who has eve ridden against the wind). The air drag plays usually the dominant role in cycling, rolling resistance is less important but still important enough.
This is only for a bike that is traveling on the plane and has reached it's final speed. In order to reach your final speed, you have to accelerate the extra mass (9% more). A heavier bike is also a problem when climbing (complete system is 9% heavier).

(03-16-2010, 11:26 AM)Joe_W Wrote: Once you have reached the speed, the only energy needed is to overcome .....

Interesting and complicated but looks you know things better. yes air pressure i s important. aerodynamics come into play when speed is 27km i know. but not sure how extra weight affects the rides.
I was just curious though. an extra 7 kg on back for instance will only reduce 1 min in 24 km/ride per hour is hard to digest though.

(03-16-2010, 11:26 AM)Joe_W Wrote: Once you have reached the speed, the only energy needed is to overcome .....

Interesting and complicated but looks you know things better. yes air pressure i s important. aerodynamics come into play when speed is 27km i know.

[ rant ]
Where do people get such a number from? It sounds as if pulled from the nether regions... Until somebody can show me the numbers for air drag coefficient and rolling resistance I doubt any "comparison". Sorry, but I see those numbers cropping up, they might be a rough estimate at best but until somebody shows me the calculations I would not use it.
[ /rant ]
Sorry. I did my PhD on this (well, other stuff, but still physics). People using numbers that are not based on measurements or estimates I have seen or at least told where to find make me angry.

Quote: but not sure how extra weight affects the rides.
I was just curious though. an extra 7 kg on back for instance will only reduce 1 min in 24 km/ride per hour is hard to digest though.

Thanks for the response

Makes you think, eh? So all those guys and gals always in need of the latest and greatest and lightest... well, should look at the numbers. Also note: I can probably save about 2-6 kg mass on my own body (I really like eating, so that is not going to happen), what sense is there in getting a bike that is 1 kg lighter?

Well, I find the handling so different between my new road bike and my old Peugeot. I like both, the Peugeot is set up as a cyclocross, the steel fork gives a nice comfortable ride. The frame is not really stiff though (I can see the chainwheel moving sideways when pedaling very hard), and it starts to shimmy above 50 km/h. The road bike handles much quicker, feels more "direct" and agile, I sit on it more aggressive, STI levers are also a very nice invention, I now shift much more than on the Peugeot where I sometimes spin madly or hammer hard because I don't want to reach down to the down tube shifters. I don't really want to compare them and would keep them both (though the Peugeot's frame needs a paint job badly).

Joe, I don't have any experience with new bikes so I can't really speak to the feel of the ride. But I am rebuilding, from the ground up, an old 1985 Schwinn World. After also tackling a 1978 Raleigh I can certainly see big differences between the two in quality of parts, such as the freewheel sprockets. But I am hoping when I get the Schwinn done, repainted and many parts replaced, that I might have a good quality road bike that I can truly call "my own."

Thanks for posting guys! It's not about expense! Think this way how will u perform on a one hour ride on ur fav bike with extra weight say 9 kg on ur back! Removing the aerodynamics just the drag. How much time u lose?

Since my average is "close to" 30km/h I cannot remove air drag... on my road bike . Also note that sprinting the other guy for the city limits sign will be more difficult. Well, using the numbers you gave (80kg rider, 7 kg bike, 24 km/h average, what the frigg is this guy doing on this machine?!) your average speed will drop by about 2km/h.

(03-17-2010, 08:07 AM)Joe_W Wrote: Since my average is "close to" 30km/h I cannot remove air drag...

Air drag was same for two experiments! only diff was in second session there is 9 kg extra weight to carry for full hour.
I will do this test soon and report back
but this will not be scientific coz i wont be feeling same on two occasions and air pressure is another factor and mindset. until then i will keep guessing how much it affects.
Interestingly the team leader in team races don't carry much drinks and are helped so it should be something more.

Well, air drag will not be the same for both experiments unless you travel at the same speed. The only way to judge the influence of the extra weight would be the use of a power meter. If you don't have one, the experiment can not be done, at least not in any scientifically sound way.

I find the biggest difference between a 19 lb. bike and a 31 lb. bike is in the effort required to flip it and lift it overhead onto my ceiling mounted hanger hooks!

(03-16-2010, 05:54 PM)Joe_W Wrote: Well, I find the handling so different between my new road bike and my old Peugeot. I like both, the Peugeot is set up as a cyclocross, the steel fork gives a nice comfortable ride. The frame is not really stiff though (I can see the chainwheel moving sideways when pedaling very hard), and it starts to shimmy above 50 km/h. The road bike handles much quicker, feels more "direct" and agile, I sit on it more aggressive, STI levers are also a very nice invention, I now shift much more than on the Peugeot where I sometimes spin madly or hammer hard because I don't want to reach down to the down tube shifters. I don't really want to compare them and would keep them both (though the Peugeot's frame needs a paint job badly).

Ditto. I never realized how seldom I changed gears until I rode a bike with integrated shifters. Now I shift continuously and my cadence is ~locked on.

(03-19-2010, 01:24 AM)j beede Wrote: I find the biggest difference between a 19 lb. bike and a 31 lb. bike is in the effort required to flip it and lift it overhead onto my ceiling mounted hanger hooks!

awesome comment!

madcyclist Wrote:Interestingly the team leader in team races don't carry much drinks and are helped so it should be something more.

Well in road races you usually draft, so now weight does become an important issue.

Man thats a lot of info!!!!! But it goes without saying the lighter the better 'especially' when it comes climbing, and lets face it there ain't too many roads that are flat. But also in fairness it doesn't matter what bike your on if u can't spin the pedals forget it. So developing our own fitness and power output and controlling personal weight is the best place to start. "Just my opinion"

Ha ha very good sorry bout that! I suppose you could say much of holland is mostly flat too! The only thing is I live in ireland and the flattest piece of road I face is rolling across my kitchen floor heading out the door after that it's up and down hill from there!! Us IRISH were never good with level's so it pays not to be carrying a few extra Lbs in the tummy region!